Optically variable devices of various types are used as an efficient anti-copy means on bank notes and security documents. A large part of the world-wide printed currency relies on such optically variable copy protection devices, and among these, features printed with optically variable ink (OVI™) have acquired a preeminent position since their first appearance on currency in 1987.
Optically variable pigment (OVP) shows a viewing-angle dependent color appearance which cannot be reproduced by color copying equipment. Various different types of OVP materials are commercially available today.
Very brilliant colors are obtained with a first type of OVP, made by physical vapor deposition. This type of OVP is constructed as a thin-film vapor-deposited Fabry-Pérot resonator stack. Simple-sandwich metal-dielectric-metal, as well as double-sandwich metal-dielectric-metal-dielectric-metal layer sequences are described in the prior art. The top metal layer(s) must be partially reflecting/partially transparent, such that light can be coupled in and out of the Fabry-Pérot resonator stack.
Said optically variable thin-film material is obtained as a continuous sheet on a carrier foil. It can subsequently be detached from its carrier and comminuted into a pigment, which consists of flakes having a diameter of 20 to 30 μm and a thickness of about 1 μm. This pigment can be formulated into inks or coating compositions, preferably for screen-printing or intaglio-printing applications.
The optical variability of said pigments relies on an interference effect. Incident light falling upon an OVP flake of said metal-dielectric-metal type is partially reflected at the top metal layer and partially transmitted, travelling through the dielectric layer and reflected back at the bottom metal layer. Both reflected parts of the incident light finally recombine and interfere with each other. Constructive or destructive interference results, depending on the thickness of the dielectric layer and on the wavelength of the incident light. In the case of white incident light, some of the white light components, having determined wavelengths, are reflected, whereas other components, having other wavelengths, are not reflected. This gives rise to a spectral selection, and hence to the appearance of color.
The path difference between the top-reflected and the bottom-reflected part of the light depends noteworthy on the angle of incidence, and so does the resulting interference color.
Another, second type of OVP, is based on coated aluminum flakes. Mechanically flattened aluminum particles are coated by chemical vapor deposition (CVD) or by wet chemical methods with a dielectric layer and a subsequent metal or second dielectric layer. Interference colors result by the same effect as described above. This type of OVP is cheaper in manufacture than the first type, but it also exhibits less brilliant colors and less angle-dependent color shift than the first type.
Large amounts of “optically variable” and “iridescent” pigment are produced for merely decorative purposes (automotive paints, lacquers, toys and the like), and are thus available to the common public in the form of coating compositions. The security potential of optically variable ink features on bank notes is considerably decreased if no clear distinction can be established between “Security OVP” and “Decorative OVP”. A counterfeiter could noteworthy reproduce bank notes on a color copier and add the missing optically variable features with the help of a commercially available decorative paint or spray.
For these and other reasons, security OVP must be made materially distinguishable from the merely decorative, commercially available types of OVP. An effective way of doing this is to dope the security OVP with a covert magnetic feature. The “magnetic OVP” allows noteworthy the implementation of different levels of security into correspondingly marked documents: i) a simple “magnetic present/not present” feature; ii) identification of the magnetic characteristics of the feature; iii) a printed pattern of magnetic and non-magnetic features; and iv) a magnetic data carrier, allowing magnetic storage of information in the printed magnetic OVP feature.
Such magnetic OVP has been proposed in U.S. Pat. No. 4,838,648. A particular magnetic material is, to this purpose, incorporated into the OVP design. The OVP of U.S. Pat. No. 4,838,648 is of the metal(reflector)-dielectric-metal(absorber) multilayer Fabry-Pérot type, and has preferably a magnetic cobalt-nickel 80:20 alloy as the reflector layer. Alternatively, but less preferably, the magnetic alloy may also be present as the absorber layer. The device according to U.S. Pat. No. 4,838,648 has noteworthy the shortcomings of i) showing a degraded optical performance, in particular a lower chromaticity, due to the lower reflectivity of cobalt-nickel alloy, as compared to aluminum, and ii) the lack of freedom for choosing the magnetic material. This latter must noteworthy comply at the same time with the functions of a magnet and of a good optical reflector, and there are only very few materials satisfying both conditions.
It is a first goal of the present invention to provide security OVP which is made materially different from decorative OVP through the incorporation of particular magnetic properties.
It is another goal of the present invention to incorporate said magnetic properties into said OVP without degrading the OVP's chromaticity and color shifting properties.
It is a further goal of the present invention to provide said magnetic OVP with as large as possible freedom for selecting the magnetic material.
It is still another goal of the present invention to provide security OVP which can be manufactured using the same equipment and process that are used for the production of “common”, non-magnetic OVP, without significantly increasing the production cost.